FIG. 1 is a diagram illustrating an organic light emitting diode (OLED) device and an optical path inside the OLED device in the related art.
Referring to FIG. 1, the OLED device in the related art includes a transparent substrate 110, and an anode layer 120, an organic light emitting layer 130 and a cathode layer 140 which are sequentially formed on the substrate 110. The organic light emitting layer 130 includes a hole injection/transport layer 131 in a lower part and an electron injection/transport layer 133 in the upper part. In a general bottom emission type device, the anode layer 120 uses a transparent electrode, such as indium tin oxide (ITO), and the cathode layer 140 uses a metal having high reflectivity, such as aluminum. A sealed protection layer 150 configured to protect the organic light emitting layer 130 is formed on the cathode layer 140 or the cathode layer 140 is packaged with a glass plate, etc.
The OLED device illustrated in FIG. 1 is generally formed of a thin film having a thickness of about λ/4 (λ is a wavelength of light), and brightness, an optical spectrum and various angle dependence of the OLED device are remarkably changed by an optical phenomenon, such as multiple reflection, light interference, light absorption and light scattering generated in boundary surfaces and insides of the respective layers.
According to Thompson, et al., it is known that efficiency of an external energy indicating luminous efficiency of an organic electroluminescence device can be represented by a multiplication of internal energy efficiency of a device and light extraction efficiency, and the light discharged from the organic light emitting layer 130 fails to be discharged to the outside of the substrate 110 by total reflection, etc., and is held in the insides of the respective layers in a process of passing through the boundary surfaces of the respective layers having different refractive indexes, so that the external luminous efficiency cannot exceed 20% (Optics Letters 22, 6, 396, 1997). Light confined in the respective layers within the OLED device and waveguided inside of the respective layers is referred to as waveguide mode light, and light discharged to the outside after passing through the boundary surfaces of the respective layers is referred to as discharge mode light. Conversion of the waveguide mode light into the discharge mode light in a surface light source device in a form of a panel and emission of the converted light to the outside of the device is referred to as light extraction.
In an OLED device manufactured on a flat substrate having the refractive index of 1.5 in the related art, since light generated in the organic light emitting layer 130 is reflected in the cathode layer 140 or is discharged toward the anode layer 120, most of the generated light is finally discharged toward the anode layer 120. However, in a process of discharging the light to the air after passing through the organic light emitting layer 130, the anode layer 120 and the substrate 110, refraction or reflection is generated in the respective boundary surfaces due to the difference in the refractive indexes of the boundary surface between the organic light emitting layer 130 and the anode layer 120, the boundary surface between the anode layer 120 and the substrate 110 and the boundary surface between the substrate 110 and the air. Especially, by the Snell's Law below (Equation 1), from a medium having a high refractive index to a medium having a low refractive index, lights incident to a surface of the substrate 110 at an angle equal to or larger than a threshold angle with respect to a vertical direction of the boundary surface is totally reflected, so that the light cannot be discharged to the outside and is dissipated within the device.n1/n2=sin a2/sin a1  [Equation 1]
Here, n1 is a refractive index of a material before incidence, n2 is a refractive index of a material after incidence, a1 is an incident angle with respect to a normal line of an incident surface, and a2 is a refraction angle with respect to a normal line of an incident surface.
Particularly, a visible ray refractive index of the general organic light emitting layer 130 is in the range of 1.7 to 1.9, and a reflective index of the ITO used as the anode layer 120 is in the range of 1.9 to 2.0, so that the total reflection between the two layers is not the problem. However, since a refractive index of a general glass or the plastic transparent substrate 110 is approximately 1.5, and the organic light emitting layer 130 and the anode layer 120 are very thin to the extent of several hundreds of nm, most of the light generated in the organic light emitting layer 130 is incident at an angle to be substantially parallel to the surface of the substrate 110, so that the incident light is confined as the waveguide mode light in the anode layer 120. The quantity of the waveguide mode light confined and dissipated in the organic light emitting layer 130 and the anode layer 120 due to the total reflection greatly increases to the extent of 60% of the total light emission amount, so that approximately 20% of the light is dissipated by the internal total reflection even in the substrate 110.
FIG. 2 is a diagram illustrating the OLED device in the related art employing an improved structure in order to improve light extraction efficiency and an optical path within the OLED device.
Referring to FIG. 2, in the related art, a method of improving the light extraction efficiency by forming a light scattering layer 111 between the substrate 110 and the anode layer 120 or forming a micro lens array layer 101 under the substrate 110 is mainly used. The reason is that in order to remove the waveguide mode light, a thickness from the organic light emitting layer 130 to the air that has a refractive index of 1.0 needs to be very small, that is, 20 nm or less, but it is difficult to physically implement such a very small thickness therebetween.
For the light scattering layer 111, a method of forming unevenness by using a material having a high refractive index and a material having a low refractive index or forming a scattering layer having different sizes of particles by using nanoparticles is used. However, such a process is considerably complicated and requires high costs, and especially, several technical difficulties are generated in a process of performing planarization again after the forming of the unevenness.
When the micro lens array layer 101 is formed so as to solve the problem of the total reflection between the substrate 110 and the outside air, a method of forming a micro lens on a film having the similar refractive index to that of the substrate 110 and then attaching the film to an external surface of the substrate 110 and a method of patterning and then etching the external surface of the substrate 110, and directly engraving a shape of the micro lens are used. In this case, in order to form the material of the micro lens and an unevenness pattern to have various shapes, such as a pyramid, a pillar, a wave and other irregular patterns, a large number of lithography processes and etching processes is required, thereby causing a problem of a considerable increase in the process costs.